Liver disease is a common cause of morbidity and mortality in the United States; approximately 400,000 patients suffer from chronic liver disease, caused by a variety of etiologies, and the incidence is rising. More than 25,000 patients die each year from complications of liver dysfunction, 1700 alone while awaiting liver trans- plant. Perturbations of liver growth can cause hepatic neoplasia. In addition, as the primary metabolic organ, the liver is exposed to both environmental and endogenous toxins, necessitating ongoing repair and regeneration. Using the zebrafish (Danio rerio) model, we have successfully elucidated a highly specific regulatory role for Wnt signaling in both liver development and regeneration, conserved across vertebrate species, and established a model to discover novel therapeutics for toxic liver injury. We recently completed a chemical genetic screen to identify regulators of liver development in zebrafish; we have successfully used this approach previously to elucidate conserved modifiers of hematopoiesis, one of which is currently in a phase I clinical trial. Through this screening methodology, we have discovered that estrogen is an important modifier of liver specification and growth. Estrogen is a well-characterized transcriptional regulator, which is frequently associated with cancer progression and may correspond with response to therapy. Furthermore, xenoestrogens, both naturally occurring and manufactured compounds that mimic the action of estrogen in the cell, have been shown to modulate the development and function of reproductive organs, as well as contribute to both cancer formation and therapy. Our long-term goal is to understand the molecular and cellular mechanisms by which estrogenic compounds affect the liver. Our objective here is to characterize the functional implications of estrogen exposure on liver growth during development, in regeneration after injury, and in carcinogenesis. Our central hypothesis is that estrogen exerts time and cell-type specific effects on the liver through interaction with other signaling pathways, particularly Wnt signaling. This hypothesis has been derived from our own screening results and subsequent preliminary data as well as clinical observations and cancer studies. The rationale for our work is that a detailed understanding of the impact of estrogen on liver growth will enable recommendations regarding nutrition and exposure during pregnancy and in early childhood, and reveal potential new targets for liver cancer prevention and treatment. In Specific Aim 1, we seek to define the role, timing, and targets of estrogen signaling during endoderm specification and liver formation; these studies will make use of both chemical and genetic modification of estrogen levels and signaling over the course of development, and utilize an extensive array of phenotypic, histological and functional methodologies. In Specific Aim 2, we will investigate whether estrogenic activity has an impact on organ regeneration and cancer growth; we will use previously devised surgical and chemical models of liver injury and a zebrafish liver cancer model to examine the effect of estrogen modulation on the recovery or destruction of hepatic structure and function.